1. Field of Invention
The present invention relates to a method and an apparatus for protecting a secondary battery charged to have a capacity exceeding predetermined value.
2. Brief Description of Prior Arts.
A secondary battery used as an electric source for electronic machines and instruments such as video cameras, personal computers and the like must be made smaller in size and at the same with a larger capacity, particularly as such electronic machines and instruments become higher in quality and smaller in size themselves.
A lithium-ion secondary battery has been developed as such a secondary battery fulfilling the above-mentioned requirements, as described in the magazine "Nikkei Electronics" dated April 26th in 1993(No.579).
However, it has become apparent ,that if such a lithium-ion secondary battery is nearly in a full charge state and is left for a long time, the characteristic of the lithium-ion secondary battery is degraded, and if the lithium-ion secondary battery is fully charged thereafter, it cannot recover its former capacity. For example, if such a lithium-ion secondary battery in the full charge state is left at room temperature for a month, the capacity of the same is reduced by 7 to 8% and finally as much as 20 to 30%.
Therefore, an object of the present invention is to provide a method and an apparatus for controlling capacity reduction of a secondary battery which is otherwise caused when the battery is left for a long time.